1. Field of the Invention
The present invention relates to an electronic musical instrument which is adapted so that the compression of a harpsichord, percussive, or similar musical tone, provided in digital form, does not involve the generation of quantizing noise in its small amplitude portion.
2. Description of the Prior Art
A variety of systems have been proposed for reproducing complicated waveforms of natural musical tones by means of an electronic musical instrument. Among them there is a method which stores an A-D converted waveform as it is and reads it out for reproduction of a musical tone. Usually, a 12 to 16 bit quantization is needed for preventing the generation of quantizing noise in the A-D conversion of a natural musical tone. On the other hand, the quantized musical tone is stored in a ROM of an LSI in an 8-bit unit; accordingly, a musical tone expressed by, for example, 16 bits must be compressed to an 8-bit form. However, if the musical tone is compressed as it is, quantizing noise is generated markedly in its small amplitude portion in particular. To avoid this, various data compression methods have been proposed, but they mostly involve complicated circuit structures and are not suitable for fabrication as an LSI.
FIG. 1A shows an example of the waveform of a percussive sound. For example, where it is represented by 8-bit precision, the attenuated portion of the waveform assumes an amplitude of low-order 2 to 3 bits at most, and accordingly the bit precision of the waveform amplitude lowers. That is, even if the absolute quantization precision is 8-bit, the bit precision decreases at the rising and falling portions of the waveform relative to its amplitude, by which a bit change occurs in a large ratio, producing quantizing noise. Incidentally, the human hearing becomes sensitive as the attenuation proceeds. In order to retain the quantizing noise within the allowed limit in terms of the human hearing, quantization with 16 bits or so is needed. However, when a musical waveform quantized by 16 bits is compressed to an 8-bit form for storage in the 8-bit ROM, the quantizing noise will naturally become noticeable as referred to above.
To solve this problem, according to the present invention, when the musical waveform is compressed to the 8-bit form for storage in the ROM, its amplitude is expanded, in accordance with an envelope waveform to be appended later, to such an extent that quantizing noise will not become noticeable in the attenuated waveform. This amplitude-expanded waveform is compressed to its initial form later when it is appended with the envelope waveform.